Laser scanning systems are well known in the art of optical inspection. Such systems are used particularly for detection of defects on substrates, such as both patterned and unpatterned semiconductor wafers. As the laser beam is scanned over the surface of the substrate, the radiation scattered from the surface is measured, typically using an array of optical detectors. Defects on the surface, such as foreign particles and scratches, generally scatter more strongly than clean, flat surfaces, and can thus be located and identified on unpatterned substrates simply on the basis of their scatter. On the other hand, if the surface is patterned, light typically scatters not only from defects, but also from features of the pattern itself. A variety of methods have therefore been developed for distinguishing scattered light due to defects from background scatter arising from the pattern.
Some types of features scatter light with a distinctive spatial signature. For example, long, narrow features, such as metal lines, grooves and some polycrystalline grain boundaries, scatter preferentially in a narrow angular band perpendicular to the scattering feature. As another example, square corners in a pattern tend to scatter in two preferred directions, separated by 90°. A variety of different feature types and their characteristic scattering patterns are described in U.S. Pat. No. 4,731,855, whose disclosure is incorporated herein by reference. In the system described in this patent, defects on a surface are identified by scanning the surface with a laser beam at normal incidence, and then detecting scattered light using an array of detectors positioned at different angles. The pattern of the scattered light is processed in order to discriminate between the characteristic, normal scattering patterns of desired surface features and abnormal scattering due to defects. In other defect detection systems, an image is formed of light scattered from the surface. The image is then processed to separate the defects from the background scattering.
In the course of inspecting a substrate such as a semiconductor wafer at high resolution, a very large volume of scattering data is collected. Substantial computing resources are required to process all these data digitally in order to separate defect signals from background scatter, as is performed in systems known in the art. Furthermore, when the background scatter is strong, it can overwhelm weak scattering signals from faint defects. There are methods known in the art for enhancing or suppressing features on the wafer by spatial filtering. An exemplary method is described in U.S. Pat. No. 6,366,352, whose disclosure is incorporated herein by reference. Such methods, however, are generally incapable of filtering out randomly-oriented features, and are useful only for those features having a fixed and predetermined orientation on the substrate.